The sale of $19,000 in ketamine and fentanyl — the potent synthetic drugs often mixed with heroin — to undercover cops in Great Neck led authorities to a massive operation believed to be supplying the entire East coast with drug paraphernalia, officials said Wednesday.

The suspects, who include a father-son duo and another man, set up a website selling drug packaging materials, which is legal. But after undercover cops bought some glassine envelopes — typically used to package heroin and crack cocaine — they were offered hard-core drugs like fentanyl and ketamine, both strong tranquilizers that in recent years have been mixed with heroin and can prove deadly in tiny amounts, authorities said.

In a seven-month joint operation between the Drug Enforcement Administration, and the Nassau police and district attorney, authorities on Wednesday arrested the three men and raided three storage facilities in Queens, finding a trove of packaging materials, including 2.5 million glassine envelopes, used to package heroin, marijuana and crack cocaine, police said.

“The amount of fentanyl there could take out a whole neighborhood,” said Nassau Deputy Police Commissioner Patrick Ryder, referring to the lethal nature of the drug sold to the undercover officers.

Almost 500 people died from opioid overdoses on Long Island in 2016, the most ever, with fentanyl leading the way, according to a report by Nassau and Suffolk medical examiners released in April.

Cops also seized $25,000 in cash and are seeking another $600,000 in bank accounts linked to the suspects, police said.

Charged in the scheme is Hussein “Jay” Kaafarani, 50, of 9th Avenue, College Point, Queens. Kaafarani was charged with criminal use of drug paraphernalia. He is a native of Lebanon, is not a U.S. citizen and is in the country illegally, police said. He has applied twice for visas and has been denied. ICE has placed a detainer on Kaafarani, police said.

Get the Breaking News newsletter!

Kaafarani’s son, Ali Kaafarani, 28, of 38th Avenue, Woodside, is charged with five counts each of second-and-third- degree criminal sale of a controlled substance and criminal sale of a controlled substance in the fourth and fifth degrees. He’s also charged with second-degree criminal use of drug paraphernalia.

Mohamad Nehme, 33, of 42nd Street, Astoria, worked at a dentist’s office and allegedly stole the ketamine from there, police said. Nehme is charged with five counts each of second-and-third-degree criminal sale of a controlled substance.

All three are scheduled to be arraigned Thursday in First District Court in Hempstead. It was unclear Wednesday whether they had retained attorneys.

Nassau District Attorney Madeline Singas called the heroin trade on Long Island a “continuing epidemic that continues to devastate our communities.” Regarding the drugs sold to the undercover cops, she said: “Just a small amount of this can kill and it’s here in Nassau County.”

“Each of these enforcements put a dent in these operations,” said Singas. “Each of these make these deadly drugs harder to get and each ensures that dealers and traffickers think twice before they bring this poison into our county.”